Liquid Electrophotography
In liquid electrophotography, an electrostatic latent image is formed on a photoconductor. A liquid toner containing charged ink particles is applied to the latent image, which is developed as ink particles move via a process of electrophoresis to the latent image. The developed image is then transferred to paper or another printing substrate.
Electrophoresis is use of an electric field to move charged particles in a fluid. In electrophoresis, particle movement is typically a function of particle charge, particle mobility, and electric field strength. The force on a particle from the electric field is the product of its charge and the electric field strength (F=q×E). The viscous drag on a particle is the negative ratio of its charge times its velocity divided by its mobility (F=−q×v/μ). When a field is applied, charged particles will accelerate until the drag balances the force, resulting in a particle velocity proportional to applied field strength (v=μ×E).
Hewlett-Packard's Indigo Division produces commercial digital printing presses. The presses are based upon liquid electrophotography, using an ink (such as ElectroInk®, trademark of Hewlett-Packard corp.) containing electrically charged ink particles in an oil based liquid medium. Electrophoresis is used to develop electrostatic latent images on a photoconductive drum.
The prior electrophoretic printing process includes the following steps, which are spaced around the circumference of the rotating photoconductive drum:
1. Charging. The drum is charged, typically via a corona charging unit with grid electrode, to about −1000 volts.
2. Exposure. The electrostatic charge on the surface of the drum is discharged on illuminated portions of the drum by applying image-wise patterned light, typically via a modulated diode laser and a rotating polygon optical scanner, to about 0 volts. Unilluminated portions of the drum surface remain charged.
3. Development. A narrow gap is formed between the drum and a development electrode biased to about −500 volts. The gap is filled with ink containing a dispersion of negatively charged ink particles in an oil based liquid medium. The electric field points in one direction over the discharged areas of the drum (from electrode at −500 volts to drum at 0 volts) and in the other direction over the charged areas of the drum (from drum at −1000 volts to electrode at −500 volts). The electric field causes negatively charged ink particles to move electrophoretically away from the electrode to the discharged areas of the drum, developing the latent image on the drum. The electric field also causes negatively charged ink particles to move away from the charged areas of the drum towards the electrode, forming a negative of the image. The electrode is cleaned and these negative-image ink particles are removed.
5. Squeegee. Ink particles that have not become part of the image on the drum are removed, along with most of the oil.
6. First Transfer. The developed image is transferred from the drum to a blanket or other intermediate transfer member.
7. Drying and Heating. Dry the developed image with heat and air to remove any remaining oil and to film-form it by melting the plastic polymers in the ink particles.
8. Second transfer. Transfer the developed image from the blanket to paper or other substrate, where it fuses on contact with the (relatively cold) substrate.
9. Cleaning. The photoconductor is erased with light (typically using an LED light bar) to prevent image memory effects. The photoconductor and blanket are cleaned to remove excess oil and leftover traces of ink.
Low Voltage Electrophoresis
Charged particles move in an electric field at a velocity proportional to the product of their charge, the electric field strength and their mobility (v=μ×E). Therefore, charged particles can move just as far under low voltage conditions as under high voltage conditions, but require more time to move. It is therefore desirable that the drum and backing electrode be flexible such that these can be in contact for a significant proportion of a printer cycle if low-voltage electrophoresis is to be used.
Conventional Lithography is Difficult on Curved or Flexible Substrates
A rigid substrate is typically used during fabrication of integrated circuitry. This rigid substrate is typically a silicon wafer, although many display devices, such as thin-film-transistor (TFT) liquid-crystal display (LCD) panels, are fabricated on flat glass substrates. Typically, circuitry is fabricated as multiple layers, with each layer being defined through a separately applied and exposed layer of patterned photoresist. The patterns on each layer of patterned photoresist typically must be aligned to patterns on the prior layer so that the layers of the eventual devices are in functional relative positions.
With conventional integrated circuit photolithography, rigidity and flatness in the substrate is desirable because as a flexible substrate bends, its surface stretches. As a surface stretches, precise alignment of successive patterns to prior patterns already on the substrate becomes difficult. Similarly, fabrication of semiconductor devices on a surface that is not flat poses issues with focus. Fabrication of semiconductor devices on cylindrical or flexible belt substrates is impractical with typical photolithographic processes.